For petrolheads, a car is far more exciting and generally satisfying if it makes a good noise (writes Peter Donaldson). Sound also provides important cues for drivers when controlling cars, combining with visual, acceleration and tactile cues to provide information on what the machine is doing. Vehicles that are too quiet can also be dangerous to pedestrians, who might not hear them approaching.
These issues have led the industry, perhaps inevitably, into the controversial area of artificial sound, partly to increase driver appeal and partly to meet regulations that demand augmented exterior sound from EVs to improve safety in urban areas, particularly for visually impaired pedestrians.
Simply using a car’s natural sound is becoming more difficult as pass-by noise regulations grow tighter, and the increasingly widespread use of turbocharging to improve performance and fuel efficiency mutes IC engines.
Hybrid and EV power plants are even quieter, and other acoustic sources such as tyre roar don’t become noticeable until speeds rise, so they’re of little use for warning pedestrians and they are not considered pleasant by drivers or passengers.
Purists often denigrate car manufacturers’ efforts to bring engine sounds into the car, particularly when they are computer generated. That’s often for good reason, as some examples sound as fake as they are.
Realistic Augmented Sound by Ricardo (RAS-R) is an attempt by the renowned engineering consultancy to bring authentic, enjoyable and informative sound back to modern cars that they have developed for both electric and quiet IC-engined vehicles. Rather than synthetically create the ‘soundtrack’ they want, they sense the real sounds made by the electric motor or the engine, filter and enhance them, and feed them live into the cabin and/ or external speakers.
There are several sources of noise in electric motors. They range from the mechanical and aerodynamic, such as bearing rumble, gear whine and shaft end float and cooling fan flows, to the electromagnetic, such as cogging torque and consequent torque ripple.
Cogging torque is the result of magnetic interaction between the poles of the permanent magnets in the rotor and the steel laminations in the teeth of the stator. When the rotor and stator poles line up, it takes a force to break the attraction: this force is the cogging torque.
The result is a variation in the torque on the shaft as the motor spins, known as torque ripple. Engineers normally try to minimise this, but when enhanced it can provide the raw material for the sounds they want to create.
Matthew Maunder, an engineer with Ricardo UK, has described the EV work in an SAE paper. He says the system consists of an accelerometer that is attached to the electric motor casing, which streams a signal from the power unit that is modulated in volume and frequency content according to acceleration demand from the driver’s use of the accelerator pedal and vehicle speed signals from sensors on the wheels.
This taps into what he calls the existing language of car sound while enabling its harmonic content to be filtered, enhanced and tuned, although the flexibility of output is limited by the source of the sound.
The company has also developed a set of software tools that can be used in the car to tune the gain map, filter parameters and other controls. That enables engineers to present a set of candidate sound schemes to senior stakeholders for selection.
So what does it sound like? Ricardo has short videos on its website that present a back-to-back comparison of an electric car being driven with and without RAS-R switched on. In the example without RAS-R, the dominant sounds are tyre and wind roar, but with it the electric motor becomes increasingly noticeable as the car accelerates.
As would be expected, it is a rising whine, but the pitch is comfortable and it is modulated with a slight warble for an overall effect that, subjectively, is easy on the ear and conveys speed and acceleration cues. For EV acoustics, augmented reality sounds like progress.